Emerging infectious diseases may be defined as diseases being caused by pathogens only recently recognized to exist. This group of diseases is important globally, and the experience of the last 30 years suggests that new emerging diseases are likely to bedevil us. As the global climate changes, so changes the environment which can support not only the pathogens, but also their vectors of transmission. This expands the exposure and effects of infectious disease and therefore, the importance of widespread understanding of the relationship between public health and infectious disease. This work brings together chapters which explain reasons for the emergence of these infectious diseases. These include the ecological context of human interactions with other humans, with animals which may host human pathogens, and with a changing agricultural and industrial environment, increasing resistance to antimicrobials, the ubiquity of global travel, and international commerce.